Thanks RS for the info.
If you are entering a REG it must be without the "-" i.e. GABCD.
The reason for my question is that ATC systems will couple the radar track to flight plan data and subsequently display radar label on the screen by matching the downlinked aircraft identification with the Callsign Filed ATC flight plan. If the two do not match then no radar label. So if aftrer "finger trouble" in the cockpit xxx123 is entered instead of xxx124 this could potentially cause a problem. If the aircraft identification was automatically entered in aircraft systems from the filed flight plan there would be less errors
rgds FLTID.